The n-6 family of polyunsaturated fatty acids, based on the parent linoleic acid and higher derivatives such arachidonic acid, have long been established as essential in human and animal nutrition. More recently, evidence has accumulated for the nutritional importance of the n-3 family of polyunsaturated fatty acids, based on the parent linolenic acid and higher derivatives such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These polyunsaturated acids are the precursors for prostaglandins and eicosanoids, a powerful group of compounds which produce diverse physiological actions at concentrations as low as 1 .mu.g/l. The prostaglandins are known to influence blood clotting, inflammatory and anti-inflammatory response, cholesterol absorption, bronchical function, hypertension, visual acuity and brain development in infants, and gastric secretions, among other effects.
Various members of the n-3 and n-6 families of polyunsaturated acids are natural constituents of many foodstuffs. However these polyunsaturated acids, in particular the longer chain acids such as arachidonic acid, DHA, and EPA, are either intimately combined with undesirable components such as cholesterol or are unsuitable for food applications in their functional form.
In many natural vegetable and animal derived lipid mixtures the polyunsaturated fatty acids such as arachidonic acid, DHA, and EPA are found predominantly in the phospholipid fraction of the lipid mixture and may be recovered from phospholipid concentrates. There are numerous methods in the literature for recovering phospholipids from lipid mixtures. For example, U.S. Pat. No. 4,698,185 discloses a method of separating phospholipids from crude vegetable triglyceride mixtures. The method involves the addition of water in a mass ratio about equal to the mass of phospholipids present in the lipid mixture, with or without heating, and with or without co-addition of citric or phosphoric acid, to cause the phospholipids to hydrate and separate into a second phase.
Such degumming methods, however, were designed for the removal of 1 to 2 weight % phosopholipids from crude vegetable triglycerides and are not directly applicable to the purification of other natural lipid mixtures, such as egg yolk lipids because of the order of magnitude higher levels of phospholipids (30-40 wt %) in egg yolk lipids. Addition of a 1:1 mass ratio of water to phospholipid with large amounts of phospholipids present causes the formation of a stable emulsion which prevents phase separation. Moreover, sterols tend to partition between both the phospholipid and triglyceride phases. A clean separation of sterols from phospholipids is not possible. In addition, the phospholipids are natural surfactants and as such cannot be used directly in place of the triglyceride portion of a food preparation because of the differences in food functionality between an oily triglyceride and a surfactant phospholipid molecule.
Cholesterol and other sterols, and phosphorous compounds are natural constituents of animal and vegetable lipid mixtures. However, the presence of large amounts of cholesterol and other sterols, and phosphorous in the human body is considered by physicians to be deleterious, since cholesterol has been implicated as a factor in a number of diseases, especially atherosclerosis, in which deposits containing a high proportion of cholesterol are deposited in blood vessels. Cholesterol, however, is found in significant quantities in a wide variety of foodstuffs, being present in most animal fats and eggs, and consequently restrictions upon the cholesterol intake of patients necessitate prohibiting or greatly reducing the consumption of many foodstuffs which may introduce complications in ensuring that the patients receive a properly balanced diet meeting all nutritional requirements.
In order to help people to reduce their cholesterol consumption without major modifications in their diet, it is desirable to provide a process by which cholesterol and other sterol compounds (many of which can be metabolized to cholesterol or its derivatives) can be extracted from various foodstuffs, thereby producing low-cholesterol versions of such foodstuffs. However, the process must not introduce into the foodstuff any material which is not generally recognized as safe for use in foodstuffs. In addition, the process should remove from the foodstuff not only cholesterol itself but also cholesterol derivatives and other sterol compounds which can be metabolized in the body to cholesterol or derivatives thereof, and which thus affect cholesterol levels in the body. Furthermore, the process should leave the foodstuff in a form which is as close as possible to that of the original, high-cholesterol foodstuff. Finally, the cholesterol-removal process should not remove vitamins and other important nutrients of the foodstuff.
Numerous attempts have previously been made to provide a cholesterol-removal process which meets these exacting criteria. U.S. Pat. No. 4,692,280, discloses a process for the purification of fish oils in which the oil is extracted with supercritical carbon dioxide to remove cholesterol, together with odoriferous and volatile impurities. Such carbon dioxide extraction processes, however, suffer from the disadvantage that they must be operated under pressure to keep the carbon dioxide in the supercritical phase, which increases the cost of the apparatus required. In addition, such carbon dioxide extraction processes are not very selective in the removal of cholesterol, and thus remove valuable constituents of the foodstuff. In addition, the properties of some foodstuffs may be altered disadvantageously by contact with supercritical carbon dioxide; for example, in some cases the carbon dioxide removes flavoring and odiferous components which affect the taste and smell of the treated foodstuff.
U.S. Pat. No. 5,091,117 discloses a process for removing at least one sterol compound and at least one saturated fatty acid from a fluid mixture by contacting the fluid mixture with an activated charcoal. U.S. Pat. No. 5,091,117 states, however, in column 12, lines 4-19, that the process should not be used for removing cholesterol from materials, such as egg yolks, which contain a combination of cholesterol and proteins, since a significant adsorption of proteins and their constituent amino acids occurs on the charcoal.
British. Pat. No. 1,559,064 discloses a process for removing cholesterol from butter triglycerides by distillation. However, Lanzani et al J. Am. Oil Chem. Soc. 71, (1994) 609! determined that only 90% of the cholesterol could be removed using the process disclosed in British. Pat. No. 1,559,064 without seriously affecting the quality of the end product. Excessive time at the high temperatures needed for more complete cholesterol removal was found to cause cis-trans isomerization of the polyunsaturated fatty acids. The trans form of polyunsaturated fatty acids are considered undesirable in food products. Thus, complete removal of cholesterol is not possible by distillation.
Egg yolk is an example of a lipid mixture rich in polyunsaturated fatty acids including arachidonic acid and (all-cis)-4,7,10,13,16,19-docosahexaenoic acid (DHA) in which the polyunsaturated fatty acids are predominantly bound in the phospholipids and which contain high levels of cholesterol. It is desirable to provide a process for the manufacture of egg-derived fatty acids and fatty acid esters high in polyunsaturated fatty acids which removes cholesterol and phosphorous residues without degrading or causing cis-trans isomerization of the essential polyunsaturated fatty acids contained therein or the taste and flavor of foods prepared using such fatty acid and ester mixtures. Moreover, the process for the manufacture of the fatty acid and ester mixtures should use materials which are on the Generally Recognized As Safe (GRAS) list of the U.S. Food and Drug Administration in order for the final product to be used in foods.